Where the term “light” is used, this refers generally to electromagnetic radiation, and not specifically to visible light. Where the term “laser” is used, this refers to a semiconductor laser unless specified otherwise.
Thermally tuned semiconductor lasers (e.g. distributed Bragg reflector, DBR, lasers) are being developed to improve linewidth performance, compared to known electronically tuned lasers. Each type of tuning acts by modifying the refractive index of one or more components of the laser such as a reflector, causing that component to select for a different wavelength.
Electronically tuned lasers provide high levels of optical loss, which increases the laser threshold current and degrades the linewidth. Furthermore, because electronic tuning has a very fast response (on the order of nanoseconds), electronics noise is easily coupled to the laser output.
In contrast, thermal tuning does not significantly increase optical loss, so there is negligible degradation of linewidth. Furthermore, since the response of thermal tuning is much slower (on the order of several tens of microseconds), the laser output is decoupled from high frequency noise sources. Heat is applied to the waveguide optical core via a resistive heater stripe running on top or closely parallel to the waveguide ridge. The stripe is electrically isolated from the ridge by a passivation dielectric.
A typical electrically tuned laser has a cross section as shown in FIG. 1A. The laser comprises a p-cladding 101, a waveguide core layer 102, an n-cladding layer 103 and a substrate 104. The p-cladding 101 is etched to form a waveguide ridge 105, to which is attached electrical means for varying the refractive index (not shown). The region of the waveguide core layer under the waveguide ridge forms the waveguide core.
FIG. 1B shows a “buried heterostructure” laser. The laser comprises a p-cladding 111, a waveguide core layer 112, an n-cladding layer 113 and a substrate 114. Instead of the waveguide ridge 115, the waveguide is formed by a structure 115 in the upper cladding layer and waveguide core layer, which is isolated by an isolating region 116. The waveguide core of the buried heterostructure laser is formed by the section of the waveguide core layer within the structure 115.
Each laser is a planar structure of well heat-sunk materials, designed to extract the heat generated by the diodes. However, this means that when adapting such a laser design for thermal tuning, the power required to cause the necessary temperature shifts is very large (e.g. 1W for a 50-70° C. temperature change). To improve the efficiency of thermal tuning, it is desirable to thermally isolate the waveguide from the support structures. However, sections of the laser which are not thermally tuned should be in thermal contact with the support structures so that their temperature can be held constant.
An example known structure for achieving this is shown for a ridge waveguide laser in FIGS. 2A and 2B, where FIG. 2A is a cross section view of the structure along the line IIA-IIA in FIG. 2B, and FIG. 2B is a plan view. The laser has an upper p-cladding layer 201, a waveguide core 202, and a lower n-cladding layer 203. The upper p-cladding layer is etched to form a waveguide ridge 204. A layer of sacrificial material 205 is located between the lower cladding layer and a substrate 206, and this sacrificial material is etched out by a wet etch process to leave an airgap 208 underneath the section containing the waveguide ridge. Vias 207 are provided in the upper cladding layer, waveguide core, and lower cladding layer to allow the wet etch to reach the sacrificial material. Clearly, if the vias completely surround the waveguide, then it would no longer be supported, so support structures 209 are provided in the vias to connect the waveguide to the rest of the substrate.
However, these support structures cause the waveguide to have uneven thermal characteristics—i.e. parts of the waveguide near a support structure will cool more readily than parts distant from a support structure. This uneven heating affects the uniform control of refractive index along the component and reduces the performance of the laser.